Zinc or zinc-nickel alloy plating on an iron and steel material protects iron due to a self-sacrifice anti-corrosion effect and thus has been widely applied as a method for inhibiting corrosion of iron and steel. However, only zinc or zinc-nickel alloy is not enough in terms of corrosion resistance and thus a chromic acid treatment, so called chromate treatment, oil zinc or zinc-nickel alloy plating has been widely applied in industry. On the other hand, recently, hexavalent chromium is eluted from abandoned vehicles and home electric appliances due to acid rain and thus it is pointed out that it harms human bodies because the eluted hexavalent chromium contaminates soil and groundwater and thus affects ecosystems, and as a result, it is an urgent technical issue to provide alternatives to a hexavalent chromium coating film.
A corrosion resistance coating film in which trivalent chromium is used is proposed as one of said alternatives. For example, JP 2000-54157 A proposes a chemical conversion treatment in which trivalent chromium, phosphorus and a metal salt such as molybdenum are used. However, as a result of our confirmation test, it was found that a satisfactory black appearance and corrosion resistance could not be reproduced. In addition, JP 2000-509434 A proposes a chemical conversion treatment in which trivalent chromium, nitric acid, organic acid and a metal salt such as cobalt are contained. Since in this treatment a concentration of trivalent chromium is as high as the range of 5 to 100 g/l and the treatment is carried out at an elevated temperature, this treatment has the advantage that a good corrosion resistance can be obtained, but the disadvantage that a stable corrosion resistance cannot be obtained. In addition, since in the treatment solution, the content of trivalent chromium is high and the organic acid is used in a large amount, water disposal thereof is difficult and the amount of sludge produced after the treatment is large. This produces a large amount of waste and thus this treatment has the disadvantage that a substantial environment load is produced due to the waste. In addition, this treatment has a problem that the black and burnished appearance of the chemical conversion coating film changes significantly depending on the amount of nickel eutectoid. This treatment also has the problems that it is necessary to elevate a temperature of the treatment solution if the uniform black and burnished appearance and corrosion resistance are industrially obtained and the treatment solution has a narrower tolerance to pH and concentrations of constituents.    Patent Article 1: Japanese Un-Examined Patent Publication 2000-54157; and    Patent Article 2: Japanese Un-Examined Patent Publication 2000-509434.